A set of studies is proposed to investigate speech sound discrimination in human infants between 6 and 12 weeks of age using the high amplitude sucking (HAS) procedure (Eimas, Siqueland, Jusczyk and Vigorito, 1971) and synthetic speech stimuli. The primary objective of this research is to study the ability of young infants to discriminate contrasts in speech sound duration and in patterns of fundamental frequency (Fo) change. As yet there have been few attempts to examine duration and Fo perception in preverbal infants even though there is evidence that these acoustic properties frequently serve as linguistic cues for speech perception with adult listeners. Nested within the studies of contrastive duration and Fo will be an investigation of several factors which we hypothesize may interact with discrimination performance: the degree to which exposure to the contrast being studied has been provided to the infant by his particular language environment; the degree of embedding of the contrast within a multiple-syllable frame; the intrinsic discriminability of the contrast being studied, predicted from results of adult studies in speech perception and auditory psychophysics. In conjunction with the infant studies there will be a matching set of studies carried out with adult subjects using a procedure modeled after the HAS procedure. This will be done to provide comparable data to use in interpreting the results of the infant studies as well as to provide information, currently lacking, about adult discrimination performance with certain contrasts in speech sound duration and Fo change.